A DC-DC converter, as a power supply device for supplying voltage constantly regardless of the size of a load, which generates output voltage completely different from input voltage in order to supply voltage which the load desires and maintains the output voltage constantly regardless of fluctuation in the input voltage.
The DC-DC converter is various in a configuration and an operating mode thereof. In general, the DC-DC converter uses a transformer which is capable of changing voltage by using a winding rate of a coil. However, in an application field in which voltage is relatively low, a DC-DC converter integrated by an IC is used. In addition, the operating mode of the DC-DC converter is determined by configuring topology such as a buck, a boost, or a buck-boost by determining a connection configuration of a switch and a transformer or an inductor constituted by transistors according to the relationship between magnitudes of the input and output voltages.
Meanwhile, as a method of controlling the switch, a pulse width modulation (PWM) mode is most widely used. In this mode, as the switch is operated in synchronization with a clock having a predetermined cycle, which is given inside or outside, and energy stored in an inductor is repeatedly increased and decreased periodically, and as a result, power is transferred from an input to an output. In this mode, the output can be stabilized to a desired value by repeatedly turning on and off the switch at all times during synchronization with the clock, that is effective under a condition that it is difficult to maintain the output voltage when the load is large.
On the contrary, a pulse frequency modulation (PFM) mode is a mode of operating the switch only when necessary without continuously turning on and off the switch in synchronization with the clock is. When the PWM mode is used in the case where the load is small, power is consumed by unnecessarily turning on and off the switch, and as a result, conversion efficiency of the DC-DC converter is decreased. Accordingly, the PFM mode is effective, which operates the switch only when necessary in order to increase the conversion efficiency when the load is not large.
The PWM mode and the PFM mode have an advantage and a disadvantage complementarily. That is, in the PWM mode, it is effective to maintain constant voltage when the load is large, but unnecessary power consumption is generated when the load is small. On the contrary, in the PFM mode, it is effective to increase the conversion efficiency because the switch is not unnecessarily turned on/off when the load is small, but it is difficult to stabilize the output voltage when the load is large. Therefore, it is necessary to selectively use the PWM mode and the PFM mode according to the load in order to obtain high conversion efficiency regardless of a magnitude of the load. If the magnitude of the load is very small for most of the time in terms of an operating characteristic of the load, a scheme of minimizing power in the PFM mode is required.
To this end, in the existing DC-DC converter, the PWM mode and the PFM mode are automatically converted or unnecessary power consumption is reduced while operating in the PFM mode. However, in the existing DC-DC converter, a case in which both characteristics are satisfied is not common, and when the load is rapidly increased after the load is very small for a long period of time, a DC-DC converter device in which an ultra-low power PFM mode is automatically changed to a general PWM mode is required.